Yes, that is true. There can be a benefit to putting bass traps in the middle of the walls. However, you will only be able to attack some of the axial room nodes (not all). It's still a benefit, though. The link here does a room node calculation for my current HT room:
http://amroc.andymel.eu/?l=17.5&w=11.25&h=9&ft=true&r60=0.6
If you move your mouse pointer over each of the axial room nodes (the tallest lines on the frequency chart), you'll see in the "Room 3d" graphic that some nodes will build up on the front/back walls, some will build up on the side walls and some will build up on the ceiling/floor.
Interest article I found here:
http://ethanwiner.com/density.html
Where Ethan Winer found that putting Twelve 3" 703-FRK panels in a room absorbed more bass frequencies than Six 6" 703-FRK panels. These are frequencies down in the 40-80Hfz area. Makes me think of using the 703 FRK instead, because my room is becoming a little too dead with the Rockboard 80 panels even.